CODEWITHSUNDEEP
javascript
dadadodo
dadadodo

Reputation: 347

need for an explanation for this bit of code

I came across this bit of code that I cannot understand.

arr.slice(arr.findIndex(func) >= 0 ? arr.findIndex(func): arr.length, arr.length);

I know that the "?" can be thought of as "then" and the ":" as else. I particularly don't understand what this bit arr.length, arr.length suppose to mean. If you could explain this to me I'd be thankful

here is the code in full context:

function dropElements(arr, func) {
  return arr.slice(arr.findIndex(func) >= 0 ? arr.findIndex(func): arr.length, arr.length);
}
    
// test here
var result = dropElements([1, 2, 3, 4], function(n) {return n >= 3;});

console.log(result)

Upvotes: -1

Views: 69

Answers (4)

KodeFor.Me
KodeFor.Me

Reputation: 13511

Try brake it down:

arr.slice(
    arr.findIndex(func) >= 0 ? arr.findIndex(func) : arr.length, // First argument for slice method
    arr.length  // Second slide argument.
);

Now lets try analyse the first argument of the slise:

arr.findIndex(func) >= 0 ?   // If the call of arr.findIndex(func) it is greater than 0
arr.findIndex(func) :        // Then return the result of this method call
arr.length                   // Other wise return the length of the arr

So what does this code does is, start the array slicing from the number returned from the first ternary operator to the length of the array.

Upvotes: 0

BJ Black
BJ Black

Reputation: 2521

Think of it like this:

if(arr.findIndex(func) >= 0) {
    startI = arr.findIndex(func);
} else {
    startI = arr.length;
}
endI = are.length;
return arr.slice(startI, endI);

...does that help? Basically the ?...: is acting like an if else, then the whole argument is followed by a ..., second argument. Very confusing, and I hate it when folks don't use parens to clarify order of ops.

Upvotes: 1

Niet the Dark Absol
Niet the Dark Absol

Reputation: 324610

When you need code explaining, it can be a sign that it's badly written. At the very least a line like that needs a comment explaining it.

Instead, consider rewriting:

function dropElements(arr,func) {
    var start = arr.findIndex(func);
    if( start < 0) {
        // no match
        return [];
    }
    else {
        return arr.slice(start);
    }
}

Note that the second arr.length is redundant - .slice() will slice to the end of the array if not otherwise specified.

Upvotes: 2

k102
k102

Reputation: 8079

If we break up this: arr.slice(arr.findIndex(func) >= 0 ? arr.findIndex(func): arr.length, arr.length); we,ll get: arr.slice(begin, end) where

begin = arr.findIndex(func) >= 0 ? arr.findIndex(func): arr.length and end = arr.length

So, this is going to slice from begin to the end of the given array

Upvotes: 0

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